Many optical systems contain layers of contrasting refractive index, n. This gives rise to reflections within the system, diverting incident light from its designed path, decreasing efficiency and increasing noise. More particularly, several systems contain a higher-refractive index medium surrounded by two lower-refractive index media. In some such systems the higher-index medium is thin enough so that interference between reflections at the two surfaces of the higher-index layer can take place, constructively or destructively, depending on the optical path difference.
One such system illustrated in FIG. 1 is a liquid crystal display (LCD), where one indium-tin-oxide (ITO) electrode, 102, with n≈1.95, lies between layers of liquid crystal (LC), 104, with n≈1.53, and glass or color filter (CF), 101, with n≈1.53, and the other lies between LC, 104 and a planarizing resin, 108, with n≈1.59. There is also a thin layer of polyimide (PI), 103, with n≈1.70, between the LC and each ITO electrode. There are also layers of silicon nitride, 105, with n≈2.04, between layers of silica, 106, with n≈1.55 and/or the planarizing resin. The silica and silicon nitride layers are used as dielectric and for passivation in and around the thin-film transistor (TFT) system. Glass, 107, supports the structure. This is one typical structure in use in the industry.
Reflection of ambient light at a display device undesirably reduces the contrast of the image seen by a viewer. Anti-reflection coatings (ARCs) are used on the front surface of displays, which can reduce its reflectivity to less than 1%. However, reflections arise within the device, particularly at the ITO electrodes and silicon nitride layers, which also contribute to the degradation of the image in a brightly lit room. ARCs for the outer surfaces of optical systems have been studied in detail. One is described, for example, in U.S. Pat. No. 6,207,263.
Concerning interfaces within an optical system, that which has received most attention is a single interface, between a lower-refractive index medium and a higher-refractive index medium.
U.S. Pat. No. 5,061,874 describes a layer introduced between two media, together with a roughening of the interfaces, to reduce specular reflection.
Within the field of LCDs, several specific situations have been investigated, with a view to reducing reflections. Reducing reflections at a glass-semiconductor interface is the subject of US Patent Application No. 2006/0197096A. A structure which uses destructive interference of light to make a ‘black matrix’ effective as an absorber is claimed in U.S. Pat. No. 7,167,221. US Patent Application No. 2004/0109305A describes ARCs used on the elements of an LCD backlight which are in contact with air.
WO 2004/044998 describes a design for an organic light emitting diode (OLED). All of the layers, an outermost layer and the internal, light-emitting layers, have thicknesses chosen, according to their refractive index, to result in destructive interference of ambient light in reflection.
Finally, U.S. Pat. No 7,215,075 describes a structure that reduces reflections at the cathode of an OLED, which has a higher refractive index than its surrounding media. The method according to U.S. Pat. No. 7,215,075 is to replace the single cathode layer with an even number of layers, which have alternating high and low refractive indices.